Method for cleaning filter structures in filtration installations for filtering liquid products, and a filtration installation

ABSTRACT

The invention relates to a method for cleaning filter structures of filtration installations for filtering liquid products containing a gas or gas mixture that is neutral to the product, in particular for filtering drinks containing the gas or gas mixture that is neutral to the product.

The invention relates to a method according to the preamble of patentclaim 1 as well as to a filtration installation according to preamble14.

Various different methods, and filtration installations which operateaccording to said methods, are known specifically for the filtering orfiltrating of liquid products in the form of drinks, for example forfiltering beer.

As well as the filtration installations that operate according to theconventional precoat filtration method and to which the product to befiltered (filtration material) is supplied as an unfiltrate containing afiltration aid and in which the filter structure consists essentially ofthe filter sheet formed by the filtration aid or of the filter cake, thedrinks sector is making increasing use of filtration methods andfiltration installations in which the filter structure is configured asa membrane filter structure and which require no filtration aid in thenormal sense, i.e. they facilitate a filtration-aid-free filtration.Such membrane filtration systems or filtration installations offerfundamental advantages, including:

-   -   A simple process that is easy to automate    -   Among other things, low running costs provided the product or        filtration material is easily filterable    -   A continuous mode of operation    -   Powder-free and dust-free operation by avoiding filtration aids        such as kieselguhr, polymer filtration aids etc.    -   Avoids the need for filter sludge disposal etc.

However these advantages are offset by disadvantages of membranefiltration systems/installations, including:

-   -   High investment costs    -   Long regeneration times of the filtration installation concerned    -   The filter structure cannot be adapted to fluctuating unfiltrate        qualities which would be possible with precoat filters by        proportioning the filtration aid    -   The running costs can be high if the product has poor        filterability    -   Plant availability may be poor if frequent regeneration of the        filter structure or filter elements and their membranes is        necessary.

It is also often the case that filtration using membrane filterstructures or membrane filters is not economical if because of theunfiltrate quality the permeability of the filter structure rapidlydecreases as the membranes become clogged with unfiltrate residues orwith lees. Backwashing with product or filtrate or water will at leastbe necessary in these cases; this entails a loss of time and product andis frequently unsuccessful with the result that the entire filtrationinstallation must be shut down and regenerated with a cleaning orflushing agent.

A further disadvantage is that the physical and chemical strain on thefilter elements and/or on their membranes during the regenerationprocess is many times greater than during the filtration process, inwhich the filter elements and their membranes are exposed only tominimal strain due in particular to the low rate of flow of thefiltration material. Consequently the service life of the filterelements is determined not by the number of production hours but andprimarily by the number of regeneration cycles.

It is the object of the present invention to provide a method which canachieve a simplified cleaning of filter structures in filtrationinstallations by the detaching of unfiltrate residues. A methodaccording to patent claim 1 is configured to resolve this object. Afiltration installation is the subject-matter of patent claim 14.

In the present invention, the cleaning of filter structures offiltration installations and/or the removal of unfiltrate residues iseffected by the controlled and forced and/or heightened outgassing of aproduct-neutral gas or gas mixture dissolved in the unfiltrate, i.e. bya gas bubble formation that is generated during said outgassing and thateven as it occurs causes a local ‘breaking off’ of the unfiltrateresidues, and whose gas bubbles by their motion also detach and entrainsaid unfiltrate residues.

The product-neutral gas or gas mixture can be any desired gas or gasmixture that is soluble in the liquid product concerned and does notalter or disturb the desired properties of the product, for example CO2gas, nitrogen, inert gas, oxygen etc. The product-neutral gas or gasmixture is for example a gas or gas mixture which is already present inthe product or filtration material to be filtered, which occurs forexample during the manufacture of the filtration material, such as forexample CO2 gas, and/or the product-neutral gas or gas mixture is addedto the filtration material or to its unfiltrate prior to filtering. Ifthe product already contains a product-neutral gas or gas mixture (forexample CO2 gas), then gas or gas mixture which is added is identicalwith this already present product-neutral gas or gas mixture.

In a general embodiment of the invention, the filter structure is amembrane filter structure which is formed for example by a wall oftube-like or hollow-fibre-like filter elements and which is configuredas a filter membrane, which (elements) are combined together as bundlesto form filter elements. Filtration installations having membrane filterstructures of this kind are cleaned by controlled outgassing or gasrelease, for example continuously during the entire filtration process,whereby the operating pressure of the filtration installation is held ina controlled manner below the saturation pressure of the product-neutralgas or gas mixture in the filtration material. It is however alsopossible to carry out the cleaning periodically by outgassing of theproduct-neutral gas, for example at given or preselected intervals or atintervals which are controlled on the basis of the particular productiontime and/or of the volume of product already filtered. During thiscleaning, the installation pressure is briefly reduced during the normalfiltration process, for example by 1-3 bar in the case of aninstallation pressure of between 1 and 10 bar. The duration of thispressure reduction in this case is a maximum of 10 sec, preferably amaximum of 5 sec.

Preferentially the pressure reduction is effected in such a way that amodified pressure differential occurs at the filter structure and withthe result that the pressure on the filtrate side is somewhat greaterthan on the unfiltrate side. As a result, during the periodical pressurereduction there is a flow of filtrate through the filter structurecausing an additional removal of the unfiltrate residues which areloosened by the outgassing.

Through the cleaning of a particular membrane filter structure that isachieved by the controlled outgassing of the product-neutral gas, thecycle times for the regeneration of that filter structure or of themembrane filters can be significantly extended, thereby inter aliaincreasing installation throughput and installation availability as wellas in particular extending the service life of the membrane filters orof the modules containing the said filters. A significant reduction inrunning costs relative to the volume of filtered product is alsoachieved.

In a further embodiment of the invention, the filter structure is aprecoat filter structure of a filtration installation, the latter beingconfigured as a precoat filter or candle filter. In this embodiment, thecontrolled and forced and/or heightened outgassing of theproduct-neutral gas takes place immediately before the extraction of thefilter sheet or filter cake which forms the filter structure, in orderto assist this extraction.

For the purpose of the prevent invention, the term ‘unfiltrate residues’should be taken to mean in particular the materials or lees removed fromthe product during filtration, as well as used filtration aids with suchmaterials or lees removed during filtration.

If the product-neutral gas is not desired in the filtrate and/or in theend product obtained by filtration either at all or only in reducedconcentration, then the filtrate is degassed in an installationdownstream of the filtration installation.

The invention is explained in detail below through the use of embodimentexamples with reference to the figures. In the figures:

FIG. 1 shows a simplified functional representation of a filtrationinstallation for filtering liquid products, in particular beer or likebeverages;

FIG. 2 shows a simplified partial representation of one of the filterelements that form the filter modules of filtration installation 15 inFIG. 1;

FIG. 3 shows a simplified schematic depiction of a further embodiment ofa filtration installation with a filter structure formed by a pluralityof filter candles;

FIG. 4 shows an enlarged schematic depiction of one of the filtercandles of the filtration installation in FIG. 3.

The filtration installation generally designated by 1 in FIG. 1 servesthe filtration of liquid products (filtration material), in particularof drinks and again preferentially of beer. For this purpose, filtrationinstallation 1 comprises a plurality of filter modules 2 which in themanner known to a person skilled in the art exhibit, in a housing 3, aplurality of tube-like filter elements 4 each provided as a membranefilter.

For their part, the individual filter elements 4 which form theparticular filter module 2 consist of a plurality of hollow fibres 4.1with a wall forming a membrane 5 with pores 5.1 and whose wall thicknessranges from 0.8 to 1.5 μm for example. These hollow fibres 4.1 whicheach extend over the full length of filter element 4 are combined tocreate bundles of approx. 2,000 fibres; at their ends they are fusedwith one another and with a filter element housing 4.2 enclosing theparticular bundle such that each hollow fibre 4.1 forms an unfiltratechannel 6 that is open at both ends and through which unfiltrate 7 flowsduring the filtration process, and such that inside filter elementhousing 4.2 there is formed a filtrate chamber 11 surrounding hollowfibres 4.1 which is in connection with a filtrate chamber or filtratechannel (not shown in the figures) of filter module 2, said filtratechamber or filtrate channel being common to all filter elements 4.

Hollow fibres 4.1 consist of a material suitable for filter membranes,e.g. plastic, for example PES (polyethersulfone) or PP (polypropylene).Filtration takes place at membrane 5 which forms the wall of theparticular hollow fibre 4.1. The filtrate thus obtained, and which isindicated by arrows 10 in FIG. 2, enters filtrate chamber 11 surroundingthe hollow fibre bundle and then passes into the filtrate channel offilter module 2, which in the depicted embodiment exhibits twoconnections 12 and 13.

All filter elements 4 are disposed in housing 3 parallel to one anotherand open out with the upper and lower end of their unfiltrate channels 6into chambers which in FIG. 1 form the upper and lower unfiltrateconnection 8 and 9 respectively of the respective filter module 2.

Only 2 filter modules 2 are shown in FIG. 1 for the sake of a moresimple representation. In its practical execution however, filtrationinstallation 1 exhibits for example more than two filter modules 2.Whatever the number of filter modules 2, they are arranged parallel toone another in an unfiltrate circuit. The unfiltrate circuit comprisesinter alia a line 14 which is connected with lower connections 9 andwhich feeds the unfiltrate from a source 15 that provides thisunfiltrate. In order to generate the required flow of unfiltrate throughfilter modules 2 as indicated by arrow A during filtration operations, afeed pump 16 for example is provided in line 14. In line 14 there isalso provided a proportioning circuit 17 through which if necessary aproduct-neutral gas or gas mixture from a source 18 which provides saidgas or gas mixture can be added to the unfiltrate in a proportionedmanner. The product-neutral gas or gas mixture is for example CO2 gas,nitrogen, inert gas, oxygen etc.

In line 14 there is moreover provided a pressure sensor 19 whichmeasures the current pressure of the unfiltrate in line 14, i.e. on theunfiltrate side of filter modules 2 and filtration installation 1, anddelivers to a control unit 20 a measurement signal which corresponds tosaid pressure. The upper connections 8 of filter modules 2 are connectedby a line 21 through which the unfiltrate is returned to source 15 afterflowing through filter modules 2.

The filtrate that accumulates in filter modules 2 on the filtrate sideis passed to a further use and/or treatment through a filtrate line thatis connected to connections 12 and that exhibits a feed pump 23.Filtrate line 24 is also provided with a pressure sensor 25 which sendsto control unit 20 a signal corresponding to the current pressure infiltrate line 24.

During the normal filtration process, unfiltrate residues or lees areinevitably deposited on the inner surface of the particular unfiltratechannel 6 or on the inner surface of membranes 5, as is schematicallyrepresented by deposit 26 in FIG. 2, with the unfiltrate residues orlees increasingly clogging pores 5.1 of membranes 5. For this reason,filtration installation 1 and in particular filter modules 2 must beregenerated inter alia by backwashing at certain intervals duringcontinuous filtration so as to free pores 5.1 and separate the lees andtheir deposits 26 from the inside surface of hollow fibres 4.1. Forbackwashing, connections 13 are connected to a backwash line 28 whichexhibits inter alia a feed pump 27.

As mentioned at the outset, regeneration imposes a heavy strain onfilter elements 4 and/or hollow fibres 4.1, so that the service life offilter elements 4 is determined less by the normal operating time offiltration installation 1 than by the number of regenerations. In orderto reduce the number of regenerations per volumetric amount offiltration material and so increase the service life of filter elements4, a cleaning or ‘blowing clear’ of membranes 5 or of their pores 5.1 bycontrolled heightened outgassing or releasing of the product-neutral gasor gas mixture from the product, for example from unfiltrate 7, iseffected in filtration installation 1 during ongoing filtrationoperations periodically, i.e. for example at intervals that arepredefined and if necessary dependent on the volumetric amount of thefiltered product.

For this purpose, and especially when the unfiltrate does not alreadyoriginally contain—or contain enough of—a product-neutral gas or gasmixture, said gas or gas mixture is added to the unfiltrate in mixingcircuit 17 proportioned in such a way that, allowing for the temperatureof the unfiltrate, which ranges between −2 and +5° C. for example, andallowing for the operating pressure of filtration installation 1, i.efor the pressure of unfiltrate 7 and of filtrate 10 in normal operationof filtration installation 1, there is no release, or at least noexcessive release, of the product-neutral gas or gas mixture fromunfiltrate 7 and filtrate 10.

If the product-neutral gas or gas mixture is not wanted in filtrate 6 orin the end product, then filtrate 10 is degassed in a separateinstallation connected to the filtrate outlet.

For the periodical cleaning of filter elements 4 and/or of membranes 5and of pores 5.1, the pressure in filtration installation 1 andpreferentially the pressure on the unfiltrate side of said installationis briefly reduced below the saturation pressure of the product-neutralgas or gas mixture, this being effected for example by adepressurisation in line 14 and in line 21 as well if necessary, or byan appropriate triggering of feed pump 16 and of feed pump 23 as well ifnecessary. As a result of this pressure reduction there is an increasedrelease of the product-neutral gas or gas mixture with bubbling 29including within deposit 26 formed by the lees and also inside pores5.1, so that the lees are removed from the inside surface of hollowfibres 4.1 and from pores 5.1 by being blown off by the generated gasbubbles, and are then entrained with the unfiltrate flow. When cleaningis complete the unfiltrate pressure or the total pressure of filtrationinstallation 1 is returned to saturation pressure or above.

If during cleaning only the unfiltrate pressure or the pressure on theunfiltrate side of filtration installation 1 is reduced, a pressuregradient from the filtrate side to the unfiltrate side is achieved, sothat in addition to pores 5.1 being cleaned by the outgassingproduct-neutral gas or gas mixture, a reverse flow of filtrate across tothe unfiltrate side of filtration installation 1 is brought about andwith it an additional washing of membranes 5 and/or of their pores 5.1.This reverse filtrate flow removes loosened lees from pores 5.1 and fromthe inside surface of hollow fibres 4.1 in a particularly reliablemanner.

The unfiltrate is passed to filtration installation 1 at a pressure inthe range of approx. 1-10 bar depending on product, temperature andproportion of the product neutral gas or gas mixture, in which case thepressure reduction for the cleaning of filter elements 4 ranges from 1to 3 bar for example.

During periodical cleaning by outgassing there is a brief pressurereduction, for example a pressure reduction for a period of less than 10sec, and preferentially for a period of under 5 sec.

The pressure reduction is effected for example by a simpledepressurisation e.g. on the unfiltrate side of filtration installation1, but preferentially by a selective triggering of pump 16. The lattercan be carried out in an especially simple manner in particular when themotorised drive of pump 18 is a frequency-controlled electric motor.

However it is also fundamentally possible to operate filtrationinstallation 1 in such a way that filter elements 4 are cleaned by therelease of the product-neutral gas or gas mixture in a continuousmanner. In this case for example, the product-neutral gas or gas mixtureis again added to the unfiltrate that is being fed under pressure, whilebeing proportioned as a function of inter alia the temperature of theunfiltrate, such that the unfiltrate pressure is equal to or greaterthan the saturation pressure. The unfiltrate to which theproduct-neutral gas or gas mixture has been added in this way is thenpassed across a device 30, for example a pressure reducing additionalfeed pump, to filter elements 4 at a pressure that is reduced belowsaturation pressure, so that during the ongoing filtration process thereis a continuous release of the product-neutral gas or gas mixture fromthe unfiltrate and the filtrate, especially in the region of membranes5, and hence in turn a removal—by gas bubbles 29 which are produced inthe process—of the lees from the inside surface of hollow fibres 4.1 andfrom pores 5.1 of the membranes.

It was assumed above that a product-neutral gas or gas mixture is addedto the unfiltrate for the cleaning of filter elements 4. However theproduct that is to be filtered, or its unfiltrate, often alreadycontains a product-neutral gas or gas mixture in the form of CO2 gas forexample, as is the case with drinks, especially beer. In these cases itis possible to bring about the cleaning of filter elements 4 byoperating filtration installation 1 in normal operating mode with atotal installation pressure, i.e. with an unfiltrate pressure andfiltrate pressure, which is at least approximately equal to thesaturation pressure of the CO2 gas in the unfiltrate. During theperiodical cleaning of filter elements 4 and/or of their membranes 5,the unfiltrate pressure or the total installation pressure, i.e. theunfiltrate-side and the filtrate-side pressure of filtrationinstallation 1, is reduced below the current saturation pressure by forexample again controlling the output of feed pumps 16 and 22 such that,in the manner described above, the removal of the lees from the insidesurface of hollow fibres 4.1 and from pores 5 is effected by bubbling 29of the released CO2 gas. For an especially effective removal of the leesfrom pores 5.1 and of deposits 26, it is also particularly advantageoushere to reduce only the pressure on the unfiltrate side during theperiodical cleaning of filter elements 4 so as to achieve a reverse flowof filtrate through membranes 5 by means of the pressure gradient thatexists between the filtrate side and the unfiltrate side.

During the filtration of products whose unfiltrate already contains theproduct-neutral gas or gas mixture and to which the addition of this gasor gas mixture in mixing circuit 17 is essentially not necessary, thereis still the possibility of a continuous cleaning of filter elements 4by release or outgassing of the product-neutral gas or gas mixture,whereby the unfiltrate is supplied to filtration installation 1 at apressure at least equal to the saturation pressure but the filtrationinstallation is operated at an operating pressure below saturationpressure, with the result that inter alia during the filtration processthere is a continuous release and consequently a continuous bubbling 29,including in the region of membranes 5, and hence a separation of thelees from membranes 5 and from their pores 5.1.

Irrespective of whether the cleaning of filter elements 4 by a releaseof the product-neutral gas or gas mixture is periodical or continuous,even with products whose unfiltrate already contains the product-neutralgas or gas mixture, for example CO2 gas or gas mixture, it is possibleto add further product-neutral gas or gas mixture to the unfiltrate inmixing circuit 17 so as to render the periodical or continuous cleaningof filter elements 2 particularly effective.

The removal by bubbling 29 of the lees or of the deposits 26 which theycreate is assisted in particular by the fact that, because of thedirection of flow of unfiltrate 7 and the density difference in theunfiltrate, the bubbles rise and entrain the lees that still adhere tomembranes 5 and/or that are blown out or squeezed out of pores 5.1 bythe gas or gas mixture bubbles.

The cleaning of filter elements 4 significantly improves thepermeability of the filter structure that is formed by filter elements4. The period of time between two regeneration operations and hence theservice life of filter elements 4 is also considerably improved, and thefiltration costs per unit of volume of filtered product are reduceddecisively.

FIG. 3 shows a schematic representation of a filtration installation 1 athat is configured as a candle filtration installation. Filtrationinstallation 1 a consists essentially, in the manner known to a personskilled in the art, of a boiler 31 whose interior, which duringfiltration operations at least, is tightly sealed to the exterior, isdivided into a lower unfiltrate chamber 33 and an upper filtrate chamber34 by a partition 32. At or below partition 32 is formed filterstructure 35 which in the manner known to a person skilled in the artcomprises a plurality of filter candles 36 and a filter cake that isdeposited on these filter candles and which consists of a filtrationaid.

The unfiltrate that contains the filtration aid is passed in feeddirection A to unfiltrate chamber 33 through a line 39 whichincorporates a feed pump 38.

The product flows across filter structure 35 and arrives as filtrate infiltrate chamber 34 from where it is supplied in feed direction A to afurther use through a line 41 which incorporates a feed pump 40.

The unfiltrate that is fed to unfiltrate chamber 33 may for exampleoriginally contain a product-neutral gas or gas mixture, e.g. CO2 gas,or if not the product-neutral gas or gas mixture is added to theunfiltrate prior to it being passed to filtration installation 1 a.During normal filtration the operating pressure of the filtrationinstallation 1 a, i.e. the pressure in filtrate chamber 33 and inunfiltrate chamber 34, is at least equal to the saturation pressure, andso no release or outgassing of the product-neutral gas or gas mixturefrom the product takes place, including in particular within the regionof filter candles 36 or in the region of filter cake 37 and hence alsowithin the region between the outside surface of filter candles 36 andfilter cake 37.

It is only when filter cake 27 is to be extracted for the purpose ofregenerating filtration installation 1 a that immediately prior to this,the pressure inside filtration installation 1 a is reduced to belowsaturation pressure such that a separation of filter cake 37 from thefilter candles is brought about, or at least assisted, by the resultingforced outgassing of the gas or gas mixtures from the product in generaland from the unfiltrate in the unfiltrate chamber 33 in particular, orby the gas bubbles or gas mixture bubbles 29 thereby produced.

The invention has been described hereinbefore by reference toembodiments. It goes without saying that numerous variations as well asmodifications are possible without departing from the inventive conceptunderlying the invention.

REFERENCE LIST

-   1, 1 a Filtration installation-   2 Filter module-   3 Housing of filter module 2-   4 Filter element-   4.1 Hollow fibre-   4.2 Housing of filter element 4-   5 Wall or membrane-   5.1 Membrane aperture or pore-   6 Unfiltrate channel-   7 Unfiltrate-   8,9 Unfiltrate connection-   10 Filtrate-   11 Filtrate chamber of filter element 4-   12, 13 Connection to filtrate channel of filter module 2-   14 Line-   15 Source-   16 Delivery pump-   17 Mixing or proportioning circuit-   18 Source for product-neutral gas or gas mixture-   19 Pressure sensor-   20 Control electronics-   21 Line-   23 Feed pump-   24 Filtrate line-   25 Pressure sensor-   26 Lees or deposit-   27 Feed pump-   28 Backwash line-   29 Gas or gas mixture bubbles-   30 Device for reducing the pressure of the unfiltrate-   31 Boiler-   32 Partition-   33 Unfiltrate chamber-   34 Filtrate chamber-   35 Filter structure-   36 Filter candles-   37 Filter cake-   38 Feed pump-   39 Line-   40 Feed pump-   41 Line-   A Direction of flow of the unfiltrate in unfiltrate lines 14 and 16

1-19. (canceled)
 20. A method for cleaning a filter structure of afiltration installation for filtering a liquid product containing aproduct-neutral gas, said method comprising: at least once during afiltration operation, depressurizing liquid product inside thefiltration installation to a pressure that is below a saturationpressure of the product-neutral gas contained in the liquid product,thereby causing outgassing of the product-neutral gas so as to removeunfiltrate residues from the filter structure.
 21. The method of claim20, wherein the filter structure separates the filtration installationinto an unfiltrate side and a filtrate side separated by a filterstructure, and wherein depressurizing comprises depressurizing theunfiltrate side.
 22. The method of claim 21, wherein the filterstructure comprises a membrane filter structure, and whereindepressurizing comprises causing outgassing continuously throughout theduration of the filtration operation by reducing an operating pressure,at least on the unfiltrate side, to below the saturation pressure of theproduct-neutral gas in the liquid product.
 23. The method of claim 20,further comprising adding the product-neutral gas to the liquid productbefore passing the liquid product to the filter structure.
 24. Themethod of claim 20, wherein the liquid product comprises a product thatoriginally contains the product-neutral gas.
 25. The method of claim 20,wherein the product-neutral gas comprises a gas selected from the groupconsisting of carbon dioxide gas, nitrogen gas, an inert gas, and oxygengas.
 26. The method of claim 20, further comprising feeding the liquidproduct to the filtration installation at a temperature ranging from −2°C. to +5° C.
 27. The method of claim 20, wherein depressurizingcomprises depressurizing by 1-4 bar.
 28. The method of claim 20, whereindepressurizing comprises depressurizing for a maximum time of no morethan 10 seconds.
 29. The method claim 20, wherein the filter structurecomprises a membrane filter structure, and wherein depressurizing forthe cleaning of the filter structure is effected periodically during thefiltration operation.
 30. The method of claim 21, further comprisingproviding a depressurizing device on the unfiltrate side.
 31. The methodof claim 20, wherein depressurizing comprises selectively controlling afeed pump for feeding the liquid product.
 32. The method of claim 21,wherein the filter structure comprises a precoat filter structure with afilter sheet or filter cake formed by a filtration aid, and whereindepressurizing for removing or loosening unfiltrate residues comprisesoutgassing the product-neutral gas before regeneration of the filtrationinstallation.
 33. An apparatus for filtering a liquid product thatcontain a product-neutral gas, said apparatus comprising: a filtrationinstallation having: a filter structure for filtering the liquidproduct, means for depressurizing the liquid product inside thefiltration installation at least once during a filtration operation to apressure below a saturation pressure of the product-neutral gas presentin the liquid product for the cleaning of the filter structure by way ofoutgassing of the product-neutral gas, said outgassing removingunfiltrate residues from the filter structure.
 34. The apparatus ofclaim 33, wherein the filter structure comprises a membrane filterstructure.
 35. The apparatus of claim 33, wherein the filter structurecomprises a precoat filter structure.
 36. The apparatus of claim 33,wherein the means for depressurizing comprises a controllable productfeed pump.
 37. The apparatus of claim 33, wherein the filter structureseparates the filtration installation into an unfiltrate side and afiltrate side, and wherein the unfiltrate side is configured for thedepressurization.
 38. The apparatus of claim 37, wherein the unfiltrateside is configured for a depressurization of 1-4 bar.
 39. The apparatusof claim 37, wherein the unfiltrate side is configured for periodicdepressurization lasting for no more than 5 seconds.